In an automatic transmission for a vehicle in which a desired transmission ratio is attained by changing the combination of engagements/disengagements among a plurality of friction engagement elements, the respective friction engagement elements provided in a transmission case are lubricated by lubricating oil supplied from an oil pump.
The respective friction engagement elements are arranged along an input shaft and an output shaft which are provided coaxially to each other, in the transmission case. Lubricating oil derived from the oil pump flows in the input and output shafts in an axial direction, and then is discharged in a radially outer direction from oil holes formed in the input and output shafts. Thus, the corresponding friction engagement elements are lubricated.
As a basic configuration of the friction engagement element, radially-inner friction plates and radially-outer friction plates are disposed alternately in the axial direction. The friction engagement elements provided in the transmission case include a friction engagement element whose radially-inner friction plates are supported by an outer circumference of a cylindrical wall portion of a rotor (such as a drum).
In the case of such a friction engagement element, lubricating oil discharged from the oil holes blocked by the cylindrical wall portion from flowing in the radially outer direction in the transmission case. Hence, there is a risk that the friction engagement element located outside of the wall portion is deficiently lubricated.
Therefore, it is conceivable that the wall portion is formed with a communication hole which communicates a radially inner side of the wall portion with a radially outer side of the wall portion. However, in the case that the other member such as a piston configured to press the other friction engagement element is provided radially inside of the wall portion, there is a possibility that the friction engagement element located outside of the wall portion is insufficiently lubricated.
Patent Literature 1 discloses that a piston for pressing a friction engagement element provided outside of the wall portion in an axial direction is provided with a disc portion which extends in a radially inner direction to bypass the wall portion. From an oil passage formed continuously in the disc portion and the piston, lubricating oil is supplied to the friction engagement element located outside of the wall portion.
However, if the disc portion which extends in the radially inner direction to bypass the wall portion is unable to be provided, the friction engagement element located outside of the wall portion is unable to be sufficiently lubricated.